linux-kernel - [PATCH] dmaengine: stm32-dma: configure next sg only if there are more than 2 sgs

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Message-ID: <20250605-stm32_dma_dbm_fix-v1-1-44657463d7d6@foss.st.com>
Date: Thu, 5 Jun 2025 17:53:30 +0200
From: Amelie Delaunay <amelie.delaunay@...s.st.com>
To: Vinod Koul <vkoul@...nel.org>,
        Maxime Coquelin
	<mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>
CC: <dmaengine@...r.kernel.org>, <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        Amelie Delaunay <amelie.delaunay@...s.st.com>
Subject: [PATCH] dmaengine: stm32-dma: configure next sg only if there are
 more than 2 sgs

DMA operates in Double Buffer Mode (DBM) when the transfer is cyclic and
there are at least two periods.
When DBM is enabled, the DMA toggles between two memory targets (SxM0AR and
SxM1AR), indicated by the SxSCR.CT bit (Current Target).
There is no need to update the next memory address if two periods are
configured, as SxM0AR and SxM1AR are already properly set up before the
transfer begins in the stm32_dma_start_transfer() function.
This avoids unnecessary updates to SxM0AR/SxM1AR, thereby preventing
potential Transfer Errors. Specifically, when the channel is enabled,
SxM0AR and SxM1AR can only be written if SxSCR.CT=1 and SxSCR.CT=0,
respectively. Otherwise, a Transfer Error interrupt is triggered, and the
stream is automatically disabled.

Signed-off-by: Amelie Delaunay <amelie.delaunay@...s.st.com>
---
 drivers/dma/stm32/stm32-dma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/stm32/stm32-dma.c b/drivers/dma/stm32/stm32-dma.c
index 917f8e9223739af853e492d97cecac0e95e0aea3..0e39f99bce8be8c38fe33dd0246012910243d831 100644
--- a/drivers/dma/stm32/stm32-dma.c
+++ b/drivers/dma/stm32/stm32-dma.c
@@ -744,7 +744,7 @@ static void stm32_dma_handle_chan_done(struct stm32_dma_chan *chan, u32 scr)
 		/* cyclic while CIRC/DBM disable => post resume reconfiguration needed */
 		if (!(scr & (STM32_DMA_SCR_CIRC | STM32_DMA_SCR_DBM)))
 			stm32_dma_post_resume_reconfigure(chan);
-		else if (scr & STM32_DMA_SCR_DBM)
+		else if (scr & STM32_DMA_SCR_DBM && chan->desc->num_sgs > 2)
 			stm32_dma_configure_next_sg(chan);
 	} else {
 		chan->busy = false;

---
base-commit: 3c018bf5a0ee3abe8d579d6a0dda616c3858d7b2
change-id: 20250605-stm32_dma_dbm_fix-3b86271d54d4

Best regards,
-- 
Amelie Delaunay <amelie.delaunay@...s.st.com>